1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member provided with an intermediate layer, for example, a subbing layer (bond layer) or an electrically conductive layer.
2. Description of the Prior Art
While an electrophotographic photosensitive member is made up basically of a substrate and a photosensitive layer, it is effective to insert a subbing layer between the substrate and the photosensitive layer so as to improve the adhesion thereof to each other and the coating workability of the photosensitive layer, protect the substrate, cover defects on the substrate, protect the photosensitive layer from electric breakdown, and facilitate the injection of electric charge from the substrate into the photosensitive layer.
It is known that the subbing layer has hitherto been formed of a polymer such as poly(vinyl alcohol), poly(vinyl methyl ether), poly(N-vinylimidazole), ethyl cellulose, methyl cellulose, ethylene-acrylic acid copolymer, casein, gelatin, polyamide and the like.
Requirements for the subbing layer concern, in the first place, electric characteristics thereof. Since the subbing layer is used in an electrophotographic photosensitive member, it is important that the subbing layer has no adverse effect on the electrophotographic performance characteristics. Hence the subbing layer is required to have a low electric resistance. If the electric resistance is too high, a so-called residual potential remains in the subbing layer after charging the photosensitive layer and causes fog on the resulting image.
Moreover the electric resistance of the subbing layer is required not to be affected by the variation in environmental conditions, particularly by the variation in atmospheric humidity. For instance, fog will result if the electric resistance increases appreciably with decrease in the atmospheric humidity.
While such characteristics are required for the subbing layer, there have been difficulties up to now in satisfying these requirements with a single resin layer. Accordingly, a very thin resin layer or a resin layer in which a conductive powder (a powder of metal such as nickel, copper, silver and the like) has been dispersed is used as the subbing layer. However, such a thin resin layer functions insufficiently as a subbing layer while such a resin layer containing a dispersed metal powder is inferior in surface smoothness since the metal powder contains coarse particles.
On the other hand, the electrically conductive layer has hitherto been formed from an electrolyte such as lithium chloride, sodium chloride etc., dissolved in aqueous solution of a water-soluble resin such as poly(vinyl alcohol) or methyl cellulose or from a polyelectrolyte such as a macromolecular quaternary ammonium salt or a macromolecular sulfonic acid salt dissolved in water. However, such electrically conductive layers are hardly acceptable for electrophotographic photosensitive members because the electric resistance of the layers much increases with decrease in environmental humidity. In order to cover defects on the substrate surface, the conductive layer needs to be thick and therefore the electric resistance thereof is required to be low.
Since a satisfactory conductive layer is hardly obtained from a single resin, a measure taken is to use a dispersion of electrically conductive powder in a resin. Electrically conductive powders used for this purpose include powders of metals such as nickel, copper, silver, aluminum etc., powders of metal oxides such as iron oxide, tin oxide, antimony oxide, indium oxide and the like, and carbon powders.
For binding these electroconductive powders, there are used thermoplastic resins including acrylic resin, vinyl acetate resin, vinyl chloride-vinyl acetate copolymer, linear polyester, phenoxy resin and the like. However, these resins generally have difficulties in use for the intermediate layer of electrophotographic photosensitive members because they are inferior in solvent resistance and hence attached with the solvent used in a coating liquid for forming the photosensitive layer. Accordingly, thermosetting resins are preferably used as binders for the electrically conductive layer. Such thermosetting resins include epoxy resin, urethane resin, unsaturated polyester, alkyd resin, acrylmelamine resin, silicone resin rubbers that can be hardened and the like.
The electrically conductive layer needs to meet requirements, besides for the above-mentioned electric properties, for physical properties such as strong adhesion to the substrate and to the overlying layer (e.g. the photosensitive layer) and surface smoothness and for other properties relating to production techniques such as the ability to disperse the conductive powder uniformly, coating workability under appropriate conditions, ease of the hardening, and storage stability (pot life) of the coating liquid. However, the above-mentioned thermosetting resins cannot sufficiently satisfy the above requirements for physical properties and for properties relating to production techniques, still having many remaining problems to be solved.